Notepad++ v7.5.9 is the last version where those instructions would have worked, so they obviously haven’t updated their instructions recently. You might want to contact the author. On any version v7.6.3 or later, the following should work:

Assuming there is a 64-bit version of SourceCookiefier, with a DLL called SourceCookiefier.dll, you would create the directory c:\program files\Notepad++\plugins\SourceCookiefier, and place SourceCookiefier.dll in that new directory. (Each plugin is required to be in a subfolder of plugins that matches the name of the DLL)

@PeterJones Thanks Peter.
Done that, and I still can’t see it. Also copied SourceCookifier and Config directories into c:\program files\Notepad++\plugins\SourceCookifier - still nothing.
I have restarted np++ but not rebooted.
Also brought all the files up a directory, and that did not work either.
So now what?
Ian

You didn’t mention whether you see error messages, or whether it just silently loads without the plugin showing up in the Plugins menu. If an error, that will give us useful information. If no error, then most likely it’s not really in the directory you think it is.

Also, as mentioned in the link that @Michael-Vincent posted, there isn’t an official 64-bit SourceCookifier (or, there wasn’t then, anyway), though he was able to compile his own. I would have thought if you were trying to install a 32-bit plugin on a 64-bit Notepad++, that Notepad++ would have popped up the standard “Cannot load 32-bit plugin” error message.

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before I went back and re-read the thread Michael linked to, I was working under the assumption that you had a 64-bit DLL to go with your 64-bit Notepad++. But now it’s doubtful. In which case, the remainder of my post will probably not bear fruit. Though it may tell us something as to why you’re not seeing the error message about trying to load a 32-bit plugin with a 64-bit Notepad++.

Please go to ? > Debug Info > Copy Debug Info into clipboard and paste the results here.
Then go to a command prompt, and run dir /R/S "c:\program files\notepad++\plugins", and paste the results in between two triple-squigglies, like:

Often, when you’re having trouble with getting a plugin to load, it’s a good idea to try a fresh portable version … so go to downloads, grab the 64bit zip, save to your desktop, unzip to a folder on your desktop. Then unzip the SourceCookifier into the appropriate plugins/SourceCookifier/ subdir of the portable unzip, and try running the portable-unzip Notepad++…

If you just want a visual for tags, have a look at TagsView. It too was 32-bit only but C++, so I was able to compile a 64-bit version. I was a 32-bit SourceCookiefier user, but have moved to 64-bit now and don’t use SourceCkfier anymore, rather TagsView (above) and TagLEET.